Electron and Hole Behavior in Powdered TiO2 - Photoluminescence and Infrared Studies
VIRGINIA UNIV CHARLOTTESVILLE
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This thesis reports the behavior of electrons and holes in photoexcited TiO2 powder under high vacuum conditions. We studied the effect of UV irradiation and different adsorbates, electron acceptors and electron donor molecules, on the perturbation of the band structure of TiO2. The electron-hole recombination rate in electronically excited TiO2 is influenced by band bending at the TiO2 surface and was studied by photoluminescence PL spectroscopy. In addition, we also employed infrared spectroscopy to qualitatively estimate the surface condition and to quantitatively estimate the amount of adsorbates on the surface. This thesis presents a new method for measuring charge transport between TiO2 particles using an optical method, photoluminescence spectroscopy. We found that long continuous UV irradiation of the TiO2 powder caused photoexcited electrons to percolate deep into the powder inducing changes in the surface photovoltage and band flattening as detected by PL enhancement. The main finding was that in the dark after charging by illumination, the negative charge tends to return back to the surface partially restoring upward band bending as detected by the PL intensity decrease. It was found that the relaxation of the negative charge at 140 K is a very slow process, on a minute time scale. The slow process was assigned to either electron hopping from a TiO2 surface site to a surface site or from a TiO2 nanocrystallite to a nanocrystallite in the TiO2 bed. This electron hopping process is temperature-dependent with an activation energy of 15 meV. We used the rate of the discharging process to estimate the electron mobility approximately 10-10 m2 V-1 s-1 at room temperature.
- Atomic and Molecular Physics and Spectroscopy
- Nuclear Physics and Elementary Particle Physics